As the development of nanoscale mechanical, electrical, chemical and biological devices and systems increases, new processes and materials are needed to fabricate nanoscale devices and components. Conventional optical lithographic processing methods are not able to accommodate fabrication of structures and features much below the 100 nm level. The use of self-assembling diblock copolymers presents another route to patterning at nanometer dimensions. Diblock copolymer films spontaneously assemble into periodic structures by microphase separation of the constituent polymer blocks after annealing, for example, by thermal annealing above the glass transition temperature of the polymer or by solvent annealing, forming ordered domains at nanometer-scale dimensions. Following self-assembly, one block of the copolymer can be selectively removed and the remaining patterned film used as an etch mask for patterning nanosized features into the underlying substrate. Since the domain sizes and periods (Lo) involved in this method are determined by the chain length of a block copolymer (MW), resolution can exceed other techniques such as conventional photolithography, while the cost of the technique is far less than electron beam lithography or EUV photolithography, which have comparable resolution.
The film morphology, including the size and shape of the microphase-separated domains, can be controlled by the molecular weight and volume fraction of the AB blocks of a diblock copolymer to produce lamellar, cylindrical, or spherical morphologies, among others. Another important factor in the film morphology is the affinity between the diblock copolymer and the underlying surface.
Preferential wetting interfaces tend to direct the morphology of the self-assembled film. Most surfaces have some degree of preferential wetting causing the copolymer material to assemble into lines that are parallel to the surface. However, in some applications, it is desirable to produce structures that are perpendicular to a surface, requiring a neutral wetting surface (equal affinity for both blocks (AB) of the block copolymer to allow both blocks of the copolymer material to wet the surface, and using entropic forces to drive both blocks to wet the neutral wetting surface. However, neutral wetting surfaces are relatively uncommon and often require that the surface of the material layer to be modified to provide a neutral wetting interface.
Neutral wetting surfaces on silicon oxide (SiOx) or silicon nitride (SiN) have been provided by applying a neutral wetting polymer, which is fabricated by adjusting the amount of one monomer to the other, and is wetting to both blocks of a self-assembling (SA) block copolymer. For example, in the use of a diblock copolymer composed of PS-b-PMMA, a PS-r-PMMA random copolymer (60% PS) (which exhibits non-preferential or neutral wetting toward both PS and PMMA blocks and includes a cross-linkable element) has been cast as a film onto SiOx and cross-linked using UV radiation or thermal processing to form a neutral-wetting mat that loses solubility and adheres to the surface but is not chemically bound or grafted to the surface.
Additional issues arise in the use of cylindrical-phase PS-b-PMMA block copolymers to form self-assembled films whereby, under a typical anneal (at about 180-190° C.), both PS and PMMA blocks wet the air-interface to produce lines of air-exposed half-cylinders that do not completely extend to the underlying substrate. Upon removal of the half-cylinder polymer block (e.g., PMMA) to form an etch mask or template, the underlying polymer matrix (e.g., of PS) must then be etched to expose the underlying substrate to be etched.
A prospective alternate material for forming a self-assembling polymer film is poly(styrene-b-ethylene oxide) (PS-b-PEO) diblock copolymers, which have been shown to be less defect tolerant (i.e., form larger crystalline grains) than PS-b-PMMA with better ordering. Cylinder-forming PS-b-PEO diblock copolymer materials have been used to produce perpendicular oriented and highly ordered, hexagonally close-pitched cylinders that orient perpendicular to surfaces via solvent annealing of the copolymer layer. Solvent annealing caused initial domain segregation at the film-air interface with both polymer blocks wetting the air interface, which was driven downward toward the underlying substrate as the solvent evaporated and the film dried.
However, because the substrate interface is somewhat preferential wetting, a layer of the minority polymer block is formed over the substrate, which prevents the polymer domains from completely extending from the film-air interface to the substrate itself. In addition, the use of solvent annealing to form either perpendicular cylinders or parallel lamella produces the same structure universally over the substrate, which is undesirable in many applications.
It would be useful to provide a method and system for forming self-assembling polymer films such as PS-b-PEO that overcome existing problems.